The invention relates to a power converter according to the principle of the hot-air engine (Stirling engine, Ericsson engine), comprising a cylinder containing a main piston and a displacement piston, each of which can be moved forward and backward on the axis of the cylinder and each of which possesses a piston rod assembly which is connected via connecting rods with a link system to two parallel cranks which rotate synchronously in opposite directions so that the displacement piston is moved phase-shifted to the main piston.
Such a known power converter of the Stirling type has a so-called rhombic crank mechanism.
The piston rod of the main piston is tubular, and to it is attached an upper yoke which is located between the cylinder and the plane in which both parallel cranks are located. Two connecting rods, each of which is connected to one of the cranks, engage with journals in the ends of the upper yoke, so that a back and forth movement of the main piston of the cylinder and thus also of the yoke brings about a synchronous rotation of the crank shafts in opposite directions.
The piston rod of the displacement piston extends through the tubular piston rod of the main piston, and to it is attached a bottom yoke which in relation to the cylinder is located behind the plane of the cranks. At the ends of the bottom yoke, connecting rods are attached rotatably and act with their respective crank on the same crank journal as the corresponding connecting rod of the upper yoke. The bottom yoke, and thus the displacement piston, also move back and forth during the synchronous rotation of the cranks in the opposite direction, but their movement is phase-shifted in relation to the upper yoke. The phase shifting is brought about in that the connecting rods are directed from the yokes outward at an angle, and from the different yokes towards each other.
This mechanism, also called a rhombus mechanism, uses slide bearings which are lubricated with oil.
For this purpose, a special circulation lubrication system is used, comprising an oil pump and oil ducts to the various lubrication points, thus making the power converter more complicated and more expensive.
Another disadvantage is that the oil lubrication requires that the crank case of the power converter with the rhombic crank mechanism must be sealed off hermetically against the cylinder of the power converter, so that the working gases in the cylinder do not mix with oil, which could result in a harmful contamination of the heat exchanger in the power converter. Due to the necessary high pressure of the working gas, this sealing is hard to realize and requires the use of expensive and complicated seals.
Because of these disadvantages, permanently lubricated roller bearings which do not require a special circulation lubrication system are used instead of the oil-lubricated slide bearings, if possible. This applies in particular to the construction of small, one-cylinder power converters, where the cost aspect plays an important role and the lower friction of the roller bearings results in conspicuous positive effects.
With larger power converters, it is common that for the use of permanently lubricated roller bearings multi-cylinder constructions are chosen, in which the forces are distributed over a larger number of bearings which are then subject to a smaller load and thus have a significantly longer life span. But the disadvantage of such multi-cylinder constructions is again that they are significantly more expensive and complicated than one-cylinder constructions.